The present invention concerns the digitisation of analogue signals or samples of such signals for obtaining digital signals having a floating decimal point, and to the amplification of such signals or samples prior to digitisation.
The substantial progress made in recent times in the utilisation of digital information, both in the sphere of displaying such information and in that of its processing, has made itself felt in a general tendency to code in digital form the analogue signals supplied by numerous sources of widely varying nature. A typical example, to which reference will be made hereinafter, is that of seismic applications; pickup transducers, such as geophones, are distributed over prospecting sites for providing analogue seismic signals following an artifically produced quake, and these analogue signals are converted into digital signals for their utilisation.
Floating decimal point digital signals are formed of two parts, a characteristic and a mantissa, each comprising elementary binary information or bits. The notation weight assigned to each bit of the characteristic or mantissa is supplied either by the rank of this bit in time (serial digitisation) or by the fact that each bit is provided on a particular connection assigned to a predetermined notation weight (parallel digitisation), or by a combination of these two types of coding.
In a general manner, the digitisation of an analogue signal comprises taking samples of this signal which are sufficiently close together to constitute in their entirety a faithful representation of the analogue signal. Digitisation then comprises measurement of the amplitude and possibly the sign of each of these samples, the result of this measurement being expressed in the form of a digital signal. This method of digitisation comprising simply the measurement of each sample is called fixed point representation.
When the dynamic range of analogue signals is high, their digitisation with fixed point necessitates a large number of binary positions. This solution is therefore found to be complicated and difficult to employ.
To avoid these two disadvantages, it has already been proposed to carry out first of all an amplification of each sample with a controllable binary gain among a plurality of predetermined values for bringing the amplitude of each amplifier sample to the lower neighbourhood of a fixed value, called dull scale signal, and then to effect an analogue-to-digital conversion relative to this full scale signal of the sample thus amplified. This solution, called floating decimal point digitisation, is employed particularly in our French Pat. Specification No. 1,522,367.
Under these conditions, the digital signal obtained at the output of the analogue-to-digital conversion, comprising the mantissa, is a representation of the sample amplified with binary gain. This binary gain, expressed separately in digital form, is the characteristic which defines the position of the decimal point in the mantissa. In a general manner, the full-scale voltage is selected such that the binary gain is always greater than unity, and therefore the movement of the point is always made in the same direction; consequently, the sign of the characteristic remains always the same and may be suppressed.
On the other hand, the sign of the analogue signals, and therefore of the samples, is considered as forming part of the mantissa.
In the following description, "gain control" or gain ranging will be the operation consisting of amplifying a sample with an adequate binary gain to bring the amplified sample to the lower vicinity of a full-scale voltage, and "conversion" will be the operation consisting of effecting an analogue-to-digital conversion of the sample thus amplified.
Fixed point representation of analogue signals of definite polarity and rather low dynamic range forms the subject of a technical solution of considerable simplicity set forth in French Pat. Specification No. 978,054. However, the device proposed in that specification does not make it possible to digitise, in floating point, analogue signals of high dynamic range and does not possess high precision
With regard to floating point digitisation, current technique is to effect separately gain control of samples in an amplifier having binary gain control of samples in an amplifier having binary gain control by discrete values (binary gain ranging) and then to carry out the measurement of each sample thus amplified in an analogue-to-digital converter.
The two important parameters for floating point digitisation are the sampling frequency, which is related to the speed required for the electronic circuits, and the mean relative precision of measurement defined by a number of exact numerical digits, which depends particularly on the ratio between the amplitude of the amplified sample and the amplitude of the full scale signal. It was obvious that the precision of measurement may be all the greater, the higher is the number of discrete values of the gain, for analogue signals of given dynamic range.
Sample amplifiers having automatic gain control and the analogue-to-digital converters generally employed in the art are all the more complicated, the higher is the number of discrete gain values and the number of desired significant digits, respectively.
It follows more particularly that the bulk and electric power consumption of the devices of the prior art are high. When a large number of different analogue signals have to be processed, it is possible either to employ an individual floating point digitisation device for each signal, which is an expensive solution, giving rise to difficult problems when these different signals have to be compared after digitisation, or to multiplex the different analogue signals for applying them to a single floating point digitisation channel, which has the disadvantage of multiplying the speed required for this single digitisation channel by the number of simultaneouslymultiplexed analogue signals.
The two types of devices of the prior art have a bulk and electric power consumption such that it is not possible to use them wherever desired. This is a considerable disadvantage for applications (seismic applications, for example) in which a plurality of pickups are distributed over the ground, the signals provided by these pickups being ditigised on the operational sites. Under these conditions, the digitisation devices of the prior art have to be collected together in the vicinity of the sources of power, such as laboratory trucks, while the pickups are dispersed at some distance.